October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 4 8 radial lines (Figs. 1 and 2) show a “feathering” fracture mor- phology that is common in Nitinol fatigue. The overall crack front becomes locally subdivided into a number of different planes, resulting in a microscopic structure of multiple par- allel crack paths or lines as observed in Figs. 1 and 2 [7,8] . The crack propagated in a semi-circular or “thumbnail” shape approximately one third of the way (166 μm deep) through the cross section of the wire before final fracture (bottom of Fig. 1). Figure 2 is a magnification step series focusing on the bottom portion of the overall fracture sur- face from Fig. 1. The originating nonmetallic inclusion is 2.1 μm in radial length and is located within a region of high- er tensile strain (as determined by finite element analysis). No visible fatigue striations were observed at the imaged magnifications in Figs. 1 and 2. The lack of observed stria- tions is further corroborated by smooth features consistent with crack closure on the surface (see Fig. 2). Finally, Fig. 3 depicts the overload or “fast” fracture portion of the sample, where microvoid coalescence was observed. FATIGUE FRACTURE AND CRACK ARREST FROM HIGH COMPRESSIVE BENDING LOADS Nitinol may be more susceptible to damage from high compressive strains/stresses than other materials with more symmetric tension-compression behavior [9-12] . These high re- sidual tensile strains/stresses from compression can lead to the formation of shear cracks and subsequent overload frac- tures in Nitinol [11,12] . This phenomenon can also affect frac- tographic interpretation of fatigue. If compressive strains induced during crimping into the catheter are high enough, a residual tensile stress field can form after device deploy- ment/expansion and can help to initiate fatigue cracks over time. The residual tensile stress fields are mostly confined within surface bending fibers and have relatively shallow depths. However, if fatigue cracks initiate on the compres- sive bending surface (typically at a bend inner curve), fatigue crack progression can arrest when the crack front reaches the edge of the relatively shallow residual tensile stress field and enters the compressive stress field. The process of fa- tigue crack initiation, growth, and then arrest from a resid- ual tensile stress as a result of far-field compressive loads is a commonly observed phenomenon in ductile engineering FEATURE Fig. 4 — SEM micrograph that shows fatigue cracks at the intrados of a sample intentionally subjected to large compressive bending strains during a mock crimping exercise. Fig. 6 — SEM micrograph of the final fracture area of the wire apex sample on the tensile surface (extrados). A shear lip is observed at the bottom of the fracture surface. Note that the elongated micro- voids “point” back to the origin, consistent with ductile bending overload fractures in Nitinol. Fig. 5 — SEM micrographs of the lab-fractured Nitinol wire apex specimen. Top: Overview of the entire wire fracture surface. Bottom: Higher-magnification micrograph of the arrested fatigue crack orig- inating on the apex intrados. 1 2